Strength-toughness Control And Welding Property Of V-N Microalloyed Steel

With the rapid development of China's economic construction,the demand for steel materials is gradually increasing in all walks of life.The medium and heavy steel plates of microalloyed steel is widely used in engineering machinery,coal hydraulic support,bridges,ships and other fields because of its excellent strength-toughness combination.Ensuring the strength-toughness,plasticity and welding performance with low costs and green production and maximizing the benefits to meet the needs of industry.In present paper,V-N microalloyed steel is used as the research object to study the effect of different thermo mechanical control process(TMCP)on the microstructure and properties of the experimental steel,and the microstructure evolution and mechanical properties of the welded joint under different line energy conditions.The main contents and research results are as follows:(1)The low-carbon V-N micro-alloying design of experimental steel composition is adopted and harmful elements are strictly controlled.Add as few small amounts of alloying elements as possible to reduce costs without reduction in strength.The V(C,N)precipitates provide more effective nucleation points for acicular ferrite,promoting the nucleation of acicular ferrite to refine the grain.The strength-toughness of V-N microalloyed steel is improved by the combination of precipitation hardening and refinement of grain size.(2)The continuous cooling transformation(CCT)curve subjected to no deformation and 40%deformation of austenite at 900? is investigated.During different cooling rate range of 0.5?1?/s,there was ferrite phase transformation and pearlite transformation.The bainite transformation occurred in cooling rate range of 5-40?/s.Compared with the static CCT curve,the whole CCT curve is shifted to the upper-left side,because the deformation promotes the transformation of austenite and refines microstructure.Measured the critical temperatures of the experimental steel,the critical temperatures Ac1 and Ac3 are 701? and 863?,the Ar1 and Ar3 are 676? and 812?.(3)By carrying out industrial TMCP experiments with three different final cooling temperatures,the 30mm thick experimental steel(B-3)comprised of ideal and fine acicular ferrite in the thickness at finish cooling temperature of 570?.The yield strength,tensile strength,yield ratio and elongation of B-3 experimental steel are 625MPa,724MPa,0.86 and 20%respectively;the impact toughness of 149J tested at-60? was obtained.The mechanical properties of 20mm thick experimental steel(B-4)is excellent.The yield strength,tensile strength and elongation were 651 MPa,733MPa,and 18%.The impact energy obtained at-60? was 60J.The precipitation of V(C,N)can promote the nucleation of acicular ferrite,and the presence of acicular ferrite in the direction of rolling thickness makes the microstructure uniform.(4)The effect of the experimental steel structure and performance was studied.Compared to the microstructure of polygonal ferrite and pearlite,the significant enhancement in strength is primarily attributed to transformation strengthening from acicular ferrite and precipitation hardening from high fraction of 3-5 nm VC precipitates.Acicular ferrite with fine nonparallel ferrite platelets and high degree of misorientation played a determining role in enhancing toughness.(5)In the welding experiments without weld preheating and post-weld heat treatment,the yield strength,yield ratio and elongation of weld joint were?555MPa,?0.80,and?15%,respectively.And the impact energy obtained at-20? was greater than 54J.Among the gas shielded arc welded experiments of 10kJ/cm,15kJ/cm,20kJ/cm in linear energy,the welded joint showed no obvious softening.In the submerged arc welded experiment of 30kJ/cm in linear energy,the hardness value of weld and heat affected zone decreased remarkably.V-N microalloyed steel is suitable for welding below linear energy of 30kJ/cm.(6)The analysis of weld joint microstructure showed that the presence of copious nano-scale V(C,N)precipitates and acicular ferrite modified mechanical properties of coarse grain heat affect zone(CGHAZ).Especially acicular ferrite characterized by high density of dislocations can inhibit the microcrack propagation.In the fine grain area,there are a lot of fine equiaxed polygonal ferrite.The fine grain strengthening and large angle grain boundary toughening mechanism ensure the low temperature toughness of the fine grain heat affect zone(FGHAZ).